In the state of the art, it is known to obtain spirographic information. In known spirographic methods, such as described in WO2006/0182237, the concentration of a gas in exhaled breathing gas mix, often carbon dioxide or oxygen, is measured for a plurality of values of expired volumes. In this way, one obtains the gas concentration as a function of volume. From this function, one can estimate the airway dead space, which relates to a volume of breathing gas mix that is moved through the (upper) airway, such as the windpipe and bronchi, does not enter the alveoli, and hence does not take part in the gas exchange. Additionally, it relates to alveolar dead space, which is the volume of air that remains in the alveoli, even when exhaling to the max. Many methods have been designed to determine these dead spaces, such as by Fowler, in “The Respiratory Dead Space”, Am J Physiol 154, 405-416 (1948).
By means of these dead spaces, and of course other information, one can monitor the respiratory processes in a human or animal subject, in order e.g. to establish its health state and changes therein.
In practice, it has been found that the obtained information is not always sufficient or sufficiently accurate. For example, sometimes a correct diagnosis, monitoring, or adequate ventilation of the subject is not reliably possible.